Process for manufacturing strip lead frames

ABSTRACT

There is disclosed a process for producing strip material useful for making lead frames which includes preparing a ferritic stainless steel substrate as a thin strip and, before subjecting it to finish rolling, electroplating it with a first layer of copper and an outside layer of nickel or tin, after which the strip is precision rolled to final gauge. There is also disclosed the product from this process which is a precision rolled strip having a core of ferritic stainless steel and a continuous surface layer of nickel or tin with an intermediate layer of copper.

This is a division of application Ser. No. 596,039, filed July 25, 1975,now U.S. Pat. No. 3,999,955, issued Dec. 28, 1976.

BACKGROUND OF THE INVENTION

Lead frames are small metal articles that are employed to connectintegrated circuits to devices in which they are used. One of theprimary functions provided by the lead frame is to support theintegrated circuit. The integrated circuit may be attached to the leadframe by means of a silicon-gold eutectic or silver-filled epoxy bond.The integrated circuit is also usually encapsulated in a ceramic orplastic envelope. In addition to supporting the integrated circuit, thelead frame provides connections through the envelope. These connectionsare used to attach the integrated circuit to the device in which itperforms. The integrated circuit is electrically connected to the leadframe, usually by thermocompression or ultrasonic bonding very smallgold or aluminum wires between connecting points on the integratedcircuit and connecting points on the lead frame. By way of example, goldwires 1 mil in diameter may be employed, and they may be connected tothe lead frame by thermocompression bonding. After being electricallyconnected to the lead frame, the integrated circuit is encapsulated sothat leads on the lead frame are bonded to, but extend through, theceramic or plastic capsule.

To perform its many functions, the lead frame must be made of materialhaving many specific properties, among which are the following. A leadframe must be made of material that has good electric conductivity whichis necessary to transmit electrical impulses to and from the integratedcircuit. It is also necessary for a lead frame to be made of materialthat has good thermal conductivity because conducting heat through thelead frame is the major means of removing heat generated in theintegrated circuit. Fortunately, satisfying both of these requirementsis possible because thermal conductivity is directly related toelectrical conductivity according to the law of Wiedmann-Franz. Goodthermal conductivity is necessary in all lead frames but it isespecially essential in lead frames connecting high powered integratedcircuits with their environment. Lead frames must have good mechanicalproperties because they must support the integrated circuit and havegood enough properties to maintain it in the device in which it isemployed. Lead frames must have sufficient ductility to be capable ofaccepting precision forming, which involves the ability to be rolled tovery exact thicknesses and with small tolerances. In addition, leadframes must be capable of being punched or etched to patterns withexacting dimensions. Lead frames must also be made of material that hasgood corrosion resistance in order to avoid being corroded during thefabrication stages of an integrated circuit. Current manufacturingtechnology dictates that the material of a lead frame should be easy toplate with gold or silver. For example, limited portions of lead framesare plated with gold or silver in order to make adequate electricalconnections. Because the gold or silver plating is applied only tolimited portions and the external leads may remain unplated, lead framesmust be made of material that is itself easy to soft solder so that theintegrated circuits can be readily connected to other devices. Leadframes must also be made of material that seals well with encapsulatingmaterial. Thus, the surface of the lead frame must be capable of beingwetted either by the plastic that encapsulates the integrated circuit orby the ceramic material that encapsulates the integrated circuit. If aceramic material is employed to produce a hermetic capsule, it is alsoessential that the lead frame have low thermal expansion so that thermalstresses during fabrication or subsequent use will not cause thehermetic seal between the lead frame metal and encapsulant to fail. Alead frame also must be made of a material that is reasonable in cost inorder for its general use in inexpensie devices to be feasible.

Finding a material with all of these properties is difficult, and, as aresult, compromises are made. One of the more successful materials withregard to properties is pure nickel. However, precision-rolling purenickel to very thin strips with precise dimensions is very expensivebecause of high scrap losses of an intrinsically high cost material.Additionally, pure nickel has some undesirable properties for leadframes; most notably, pure nickel has high thermal expansion. An alloyof 42% nickel and the balance iron is also commonly employed inmanufacture of lead frames. The disadvantages of 42% nickel alloy arehigh cost and a relatively low thermal conductivity which necessitatessufficient thickness of high-cost silver or gold platings to raise thethermal conductivity to adequate levels. The hardness of 42% nickel alsoyields it incapable of being thermocompression bonded to theinterconnecting wires of the integrated circuit. Copper or copper alloysare also used; however, their corrosion resistance is inferior and mayin some cases preclude their use.

THE INVENTION

This invention includes a process for producing a metal strip useful forthe production of lead frames which have all of the attributes set forthabove. The process includes preparing a ferritic stainless steel, i.e.an iron-chromium alloy, in the form of a thin strip that issubstantially thicker than the final product, preferably about twice asthick as the final product. This strip is then provided with a nickelstrike to condition its surface to accept subsequent coatings. A nickelstrike is a very thin layer of nickel that is plated onto the ferriticstainless steel under conditions where the protective coating ofchromium oxide is absent from the surface of the stainless steel.

The ferritic stainless steel with a nickel strike on its surface is thensubjected to electrodeposition of copper. A layer of copper iselectrodeposited over the nickel strike to a thickness dictated by thedesirability of having a minimum of 50 microinches of copper on thesurface following cold reduction of final gauge. Preferably, the copperlayer, after cold rolling, is from about 350 to 600 microinches thick.The copper coated ferritic stainless steel is then provided with acontinuous layer of either nickel or tin of sufficient thickness sothat, after final rolling, the outer layer is at least 50 microinchesthick.

The strip of ferritic stainless steel with all coatings applied to it isthen precision cold rolled to reduce its thickness to achieve thedesired mechanical properties and a bright, smooth dense surface. In apreferred embodiment, a cold reduction of at least 50% results in ashiny strip with exact dimensions having a continuous and intact layerof nickel or tin on the outside. This strip can be precision stamped oretched to any desired lead frame configuration.

The ferritic stainless steel core provides a mechanically strong basefor the lead frame. The ferritic stainless steel has a lower thermalexpansion than either Ni, or Cu and Cu-alloys. It is much lower in costthan pure Ni, Ni-alloys, Cu, or Cu-alloys. In the form of a compositeutilizing a copper layer it is an effective heat conductor. The outerlayers of copper and nickel have their thermal expansions restrained bythe ferritic stainless steel core maintaining the lower thermalexpansion of the ferritic stainless steel. The composite having a Niouter layer has good thermal conductivity, good electrical conductivity,a surface which is similar to pure nickel lead frames in that it can beeasily plated with gold or silver. With these precious metal platings itis easy to thermocompression or ultrasonic bond and is easy to solder.The composite with a tin outer layer has all of the above attributeswithout plating with silver or gold and is believed to be easilythermocompression or ultrasonic bonded to interconnection wires. In bothcases the good characteristics of ferritic stainless steel such as highmechanical strength and lower thermal expansion are preserved, and thecomposite material has improved electrical and thermal conductivitythrough the use of the copper intermediate layer.

The composite strip with an outer layer of tin also has all of thedesirable properties of ferritic stainless steel, the electrical andthermal conductivity of copper, and the good surface characteristics oftin. Copper has poor corrosion resistance and cannot be exposed in alead frame. For example, copper must be shielded from an integratedcircuit deposited on a silicon single crystal wafer because copperdiffuses into the silicon and changes its properties. Accordingly, inaccordance with this invention, copper must always be beneath acontinuous and intact layer of nickel or tin. This circumvents one ofthe undesirable features of copper in that the nickel or tin provides amore stable, corrosion resistance surface, and one more readily bondedto other materials.

This invention also includes the product from the process describedabove. The product of the process is a strip of material having aferritic stainless steel core, an intermediate layer of copper, and acontinuous outer layer of nickel or tin; the composite strip being coldrolled, preferably between 40 and 60%, to its final thickness and havinga final thickness of less than 0.015 inches. As set forth above, theproduct of this invention has properties such that it may readily beformed into a lead frame having all of the thermal, electrical, andmechanical properties set forth hereinabove. The process of thisinvention provides a product which has properties that are equal to orbetter than more expensive lead frame materials. Very little of theexpensive materials are used and they are employed at a point in theprocess where there is little waste. The final product has excellent andcontrollable mechanical properties and a bright, smooth, shiny, densesurface that is both physically and chemically well suited for use inlead frames.

DETAILED DESCRIPTION OF THE INVENTION

The following examples are provided to demonstrate specific embodimentsof the invention.

EXAMPLE 1

A ferritic chromium-iron alloy containing 13-14% chromium and generallyknown as type 404 stainless steel was rolled to a strip of intermediatethickness (0.025 inches) and annealed. The strip was provided with anickel strike by immersing it in an aqueous solution containing 240grams per liter of nickel chloride hexahydrate and 71 milliliters of anaqueous solution of 37% hydrogen chloride. The type 404 stainless steelwas connected as the cathode, and a pure nickel anode was employed. Thebath was maintained at 80 ° F, and a current of 0.5 amperes per squareinch of cathode area was passed through the bath for 30 seconds. Thereresulted a nickel strike less than 30 microinches thick deposited on thecathode.

The specimans having a nickel strike on their surfaces were thenimmersed in an aqueous solution containing 240 grams per liter of coppersulfate and 33 milliliters per liter of 90% sulfuric acid. The type 404stainless steel was connected as the cathode, and a pure copper anodewas employed. The bath was maintained at 75 ° F, and a current of 0.152amperes per square inch of cathode area was passed through the bath for6.4 minutes. A layer of copper 295 microinches thick was deposited.

Specimens were taken from the copper bath and thoroughly washed and thenimmersed in a bath containing 38.4 grams per liter of nickel chloridehexahydrate, 273.5 grams per liter of nickel sulfate hexahydrate, and 26grams per liter of boric acid. The specimens were connected as thecathode, and a pure nickel anode was employed. Electrodeposition ofnickel was effected by maintaining the bath at 75 ° F and passing acurrent of 0.25 amperes per square inch of cathode area through the bathfor 6.4 minutes. Under these conditions, a layer of nickel 250microinches thick was deposited. The specimens had a dull finish whenthey were removed from the bath and cleaned. The specimens were thenprecision cold rolled to 0.010 inches thick after which they had amirror finish. Visual and microscopic examination indicated that thefinish was a continuous film of nickel containing no cracks, breaks orspalls.

Table I below indicates the physical characteristics of the materialproduced compared with the 404 stainless steel.

                  TABLE I                                                         ______________________________________                                                   Plating Thickness                                                                           Electrical Conductivity                              material   (microinches) (1/micro ohm cm)                                     ______________________________________                                        404 ss     --            0.0198                                               404 + Cu + Ni                                                                            40 Cu + 80 Ni 0.0257                                               ______________________________________                                    

It is evident from Table I that the layer of copper and layer of nickeldeposited on the type 404 stainless steel significantly improves theelectrical conductivity of the specimen. The nickel layer had all of thecharacteristics of pure nickel with regard to its affinity for gold,silver, solder, plastic, and ceramic materials. The composite stripexhibited substantially the thermal expansion characteristics of type404 stainless steel, and it was substantially lower than the thermalexpansion of nickel.

When the strip is punched and etched using conventional equipment andtechniques and conventional etching material, the lead frames that areproduced are sound with regard to the adherence of the plating and areas dimensionally exact as are lead frames produced by the sametechniques employing conventional materials.

EXAMPLE 2

Type 404 stainless steel was rolled and prepared with a nickel strikeand a layer of copper as set forth in Example 1. The copper coatedstainless steel was then immersed in a bath containing 26.8 ounces pergallon of Sn(BF₄)₂, 10.8 ounces per gallon of metallic tin, 10.0 ouncesper gallon of HBF₄, 3.3 ounces per gallon of H₃ BO₃, and 0.8 ounces pergallon of gelatin. The metallic tin was in the form of a powder.

The copper coated stainless steel was connected as the cathode and apure tin anode was used. The bath was maintained at 75 ° F, and acurrent of 0.11 amperes per square inch of cathode area was passedthrough the bath for 10 minutes which resulted in a layer of tin 105microinches thick. The tin plate adhered well.

The composite strip was cold rolled to 0.010 inches thick and afterrolling, a smooth continuous layer of tin, 105 microinches thick, wasthe outer layer of the strip. The composite strip had the surfaceproperties of tin, especially exhibiting excellent solderability andcorrosion resistance.

When the composite strip is punched and etched using conventionalmaterials and techniques, lead frames with excellent properties andexact dimensions are produced.

The process of this invention may be improved by employing brighteningand leveling additives in the copper plating bath. These additives causea layer of copper of uniform thickness to be deposited and therebyavoids copper penetrating the final nickel or tin coating during thecold rolling process. The use of these additives is especially indicatedwhen thicker layers of copper are deposited.

The mechanical properties of the composite strip can be controlled bythe amount of cold rolling. Preferably cold rolling should reduce thethickness about 50%, but thickness reductions between 40 and 60% willproduce adequate material. With thickness reductions in this region, theouter layers of nickel or tin should be at least 50 microinches thickafter cold rolling. The copper layer should be thick enough to raise theelectrical and thermal conductivity of the composite to the desiredlevel. Copper should be at least 50 microinches thick and preferablyabout 350 to 600 microinches thick in the final cold rolled product.

What is claimed is:
 1. A process for producing strip material useful forlead frames comprising:a. providing a strip of ferritic stainless steelnot thicker than 50 mils, b. coating all sides of said strip with alayer of copper sufficiently thick to result in a copper layer at least50 microinches thick following cold rolling, c. coating all sides ofsaid copper coating with a layer of metal selected from nickel and tinsufficiently thick to result in a layer at least 50 microinches thickfollowing cold rolling, and d. cold rolling the resultant compositestrip to reduce its thickness to below 0.03 inches; said composite striphaving a distinct layer of copper and a distinct layer of metal selectedfrom nickel and tin thereover.
 2. The process of claim 1 wherein saidmetal is nickel.
 3. The process of claim 1 wherein said metal is tin. 4.The process of claim 1 wherein said copper layer is sufficiently thickto result in a copper layer between 350 and 600 microinches thickfollowing cold rolling.